Process for treating metal



United States Patent 3,440,095 PROCESS FOR TREATING METAL Louis C. Larsonneur, Palos Heights, Ill., assignor to Nalco Chemical Company, Chicago, Ill., a corporation of Delaware No Drawing. Filed Sept. 1, 1966, Ser. No. 576,551 Int. Cl. B08b 3/08; C231? 11/04 U.S. Cl. 1343 5 Claims This invention relates to a process for inhibiting corrosive attack of aqueous solutions which are in Contact with metal surfaces. More particularly, the invention relates to the use of a 1 to 9 mole propylene oxide adduct of thiourea to protect metal surfaces from the corrosive attack of solutions of acids.

The corrosive effect of acids on metal surfaces is well known. Oftentimes, however, the use of metal surfaces requires treatment with solutions of acids to clean, brighten and generally improve the surfaces of these metals. For example, certain acids such as sulfamic acid or hydroxy substituted polycarboxylic acids are used to clean metal surfaces in air conditioning units, sea water evaporators, dairy equipment, industrial boilers, condensers and heat exchangers, food processing and brewery equipment, sugar evaporators, and various pieces of equi ment commonly used in papermaking.

Another common use of solutions of acid is generally known as the pickling process. Briefly, this well-known process comprises the use of a solution of an acid to remove various oxides (scale) which form on metal surfaces during the production and fabrication of various metals. Various acids such as sulfuric acid, phosphoric acid, hydrochloric acid, and the like are employed to clean up various metal surfaces. The pickling process is used widely in the ferrous metal production.

The seriousness of the effect of acid solutions in contact with metal surfaces is dependent upon temperature, concentration, degree of scale, dirt, etc. to be removed, and other factors. Oftentimes, it is desirable to use an inhibitor of some form which permits longer use of the acid solution without adverse corrosion of the metal being cleaned.

Accordingly, an object of this invention is to provide a process for inhibiting corrosion of metals which are in contact with solutions of acids.

A specific object of this invention is to provide an improved process for pickling metals wherein the surface defects are readily removed without substantial damage to the metal surface.

Other objects will appear hereinafter.

In accordance with the invention, it has now been discovered that 1 to 9 mole propylene oxide adducts of thiourea are suitable corrosion inhibitors which protect metal surfaces in contact with solutions of acids. Normal concentrations of the inhibitors of this invention when dissolved in acids will range from approximately 0.05% to about 5.0% based on the weight of the acid. In some instances, more or less inhibitor will be necessary, depending upon the severity of the conditions; i.e., temperature, acid strength, acid concentration and the like. Normally, it is preferred to employ about 0.1% to about 1.0% of the inhibitor of this invention.

In the cleaning step acid strength will run from about 0.1% to as high as 25% or 30% by weight. Temperatures at which the acid may be employed will range from room temperature to as high as 210 F. or higher.

The inhibitor of this invention has been found to be effective in all types of acids which are normally used in contact with metal surfaces. Specific illustrations of Cit these acids, presented 'by way of example and not by way of limitation, are phosphoric acid, sulfuric acid, hydrochloric acid, sulfamic acid, citric acid, tartaric acid, malic acid, boric acid, and the like, as well as mixtures thereof. Inhibited sulfuric acid is particularly preferred.

As stated above, the inhibitors of this invention are comprised of the l to 9 mole propylene oxide adducts of thiourea. It has been found that propylene oxide adducts of thiourea containing less than 1 or more than 9 moles of propylene oxide units are substantially less effective in protecting metal surfaces from corrosion when contacted with solutions of acids. In addition, it has been found that use of propylene oxide is substantially superior to similar cyclic oxides as an alkoxylating agent and use of this agent will result in particularly useful inhibitors for the process of this invention.

A wide variety of metals may be protected by the process of this invention. Due to the susceptibility of ferrous metals to corrosive acid attack, it is a preferred embodiment to employ the process claimed herein when ferrous metals, and especially steel are being treated with acid solutions.

To demonstrate the efiicacy of this invention, a test procedure was derived to measure the corrosive rate of various acids on various metals in the presence of inhibitors. Specifically, the procedure was as follows. A metal surface was cleaned by sand blasting and stored in refined mineral seal oil until needed. Prior to use, the metal surface was deoiled in acetone and air-dried. After weighing the sample, the metal was placed in an acid solution of known strength at a temperature of 176 F. (plus or minus one degree) for one hour. After the test, the sample was washed in saturated sodium carbonate, water and acetone and then air-dried before reweighing. In each experiment, an evaluation of the particular acid being employed was made both with and without an inhibitor of this invention. The difference in weight loss for the treated sample over the untreated sample was calculated and recorded as percent reduction in corrosion rate.

Since each test lasted one hour, and since each sample contained a known surface area, the loss of weight in milligrams was readily conveted to a corrosion rate having the units of milligrams per square inch per hour. Presented below in Table I are the result of a number of experiments on ferrous metals. In each case, the inhibitor employed was a 1 to 9 mole propylene oxide adduct of thiourea. It was found that the effectiveness of the various adducts over this range of 1 to 9 moles of propylene oxide was approximately equivalent, although those inhibitors containing 3 to 6 moles of propylene oxides were slightly superior. Attempts to inhibit the corrosive attack of the various acids with inhibitors containing less than 1 or more than 9 moles of propylene oxide reacted with thiourea were not successful. As can be seen from the data presented below in Table I, effective reduction of the corrosion rate of solutions of acid was achieved by the process of this invention.

TABLE I Concentration by weight percent Percent reduction in corrosion rate Acid Phosphoric Hydrochloric. Sulfuric The invention is hereby claimed as follows: 1. A process for inhibiting corrosion of metal surfaces 1n contact with a corrosive solution of acid which com- 3 4- prises maintaining contact of said surface with said acid References Cited which additionally contains a corrosion inhibiting amount UNITED STATES PATENTS of a 1 to 9 mole propylene oxide adduct of thlourea to substantially inhibit corrosion of said metal. 3,260,538 7/ 1966 Gardner 252F391 2. The process of claim 1 where said acid is selected 5 3,294,695 12/ 19616 Tfppett 252 391 from phosphoric, citric, sulfamic, hydrochloric and sul- 3,188,292 6/1965 Plrotte 252-391 furic acid.

3. The process of claim 2 where said metal is a MORRIS WOLK Pnmary Examiner ferrous metal. M. D. BURNS, Assistant Examiner.

4. The process of claim 3 where said acid is in contact 10 'With said metal in a pickling process.

5. The process of claim 4 where from 0.05% to 5.0% of said adduct is added, :based on the weight of said acid. 2127; 260552; 391; 13441 U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.0. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,440 ,095 April 22 1961 Louis C. Larsonneur It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 43, "conveted" should read converted line 65, "99" should read 89 Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR

Attesting Officer Commissioner of Patents 

1. A PROCESS FOR INHIBITING CORROSION OF METAL SURFACES N CONTACT WITH A CORROSIVE SOLUTION OF ACID WHICH COMPRISES MAINTAINING CONTACT OF SAID SURFACE WITH SAID ACID WHICH ADDITIONALLY CONTAINS A CORROSION INHIBITING AMOUNT OF A 1 TO 9 MOLE PROPYLENE OXIDE ADDUCT OF THIOUREA TO SUBSTANTIALLY INHIBIT CORROSION OF SAID METAL. 